The following correction was printed in the Guardian's Corrections and clarifications column, Friday December 9 2005

Our article below said in error that unregistered users are to be prevented from editing pages when it is only the creation of new pages that will be restricted to registered contributors.

The online encyclopedia Wikipedia has been forced to change the way it operates after claims it had become a breeding ground for "false and malicious" information. After a week of blunders, the operators of the site - which allows anyone to write and edit articles - are banning anonymous users from creating new entries.

"Just recently we've instituted changes," said Jimmy Wales, Wikipedia's founder. These would give the site's 600 volunteer editors a better chance of catching and removing offensive material. "The idea is basically to slow down the pace of new page creation."

The furore began last week when a journalist, John Seigenthaler, a former assistant to the former US attorney general Robert Kennedy, attacked Wikipedia in a scathing editorial in the newspaper USA Today. He was angered by an entry insinuating he had been involved in political killings. "For a brief time, he was thought to have been directly involved in the Kennedy assassinations of both John and his brother, Bobby," said the unedited biography. "Nothing was ever proven."

Seigenthaler contacted Mr Wales to get it removed. "At the age of 78, I thought I was beyond surprise or hurt at anything negative said about me. I was wrong," he wrote. "For four months Wikipedia depicted me as a suspected assassin."

Concerns were echoed by other critics, as the former MTV presenter Adam Curry, now a dotcom entrepreneur living in Surrey, was accused of "vanity editing" an entry. Mr Wales admitted there were problems with the system but said readers should be more sceptical."You should take Wikipedia with a grain of salt," he said. "It's a work in process."

The website has grown rapidly since its inception four years ago. Users are free to rewrite information, overseen by volunteer editors, based on the assumption that the wisdom of the many is more accurate than the wisdom of the few. There are now more than 850,000 articles in English alone, but while Wikipedia has been lauded for pioneering internet-based knowledge, cynics are concerned. "The real story is why would anyone presume anything written in there is accurate," said Michael Gartenberg, of Jupiter Research. "Where's the accountability?"

Registering users could result in court actions. In the long term, administrators plan to "lock down" articles that have achieved a recognisable level of accuracy.